Lipopolysaccharide (LPS) is a common component of gram negative bacteria cell walls which acts as a potent inducer of cytokine release from human monocytes. The purpose of this project is to isolate and identify the protein(s) on human monocytes which bind LPS and initiate the release of cytokines from these cells. Using sepharose beads which have been covalently crosslinked with LPS, we have isolated two proteins from human monocytes lysates, which appear to bind specifically to the LPS coated beads and not the control beads. The molecular weight of these proteins are approximately 73 and 85 kd. The 73 kd protein appears to be present in greater abundance. Two other groups, using different techniques, have likewise isolated a protein of approximately 73 kd which appears to bind LPS. Recent studies indicate that this 73 kD protein may be heat shock protein 70. Although hsp70 is found in great abundance inside cells there are some indications that this and other heat shock proteins can also be found on the surface of some cells. The addition of hsp 70 to monocytes in culture can either suppress or enhance LPS induced TNF and IL-1 release depending on the concentration of LPS used.